The present invention relates generally to pulse oximetry devices and methods. More particularly, the invention is concerned with an improved pulse oximetry system that corrects calibration shifts due to changes in monitoring site characteristics, particularly variations in blood fraction using calibration stabilization. As a result of the calibration stabilization, oxygen saturation monitoring accuracy and availability are improved. The invention is of particular value when applied in fetal pulse oximetry.
Pulse oximeters are employed in patient clinical practice as well as veterinary practice for assessing the level of oxygenation in the blood of a subject, and are well known in the art. Typically, such devices comprise a sensor with light emitting device(s) (emitter(s)) and associated photodetector(s) (detector(s)), attached to a monitoring device performing signal acquisition, analysis, and display/print and/or functions. One particular example of a pulse oximeter is described in U.S. Pat. No. 6,163,715 B1, issued to Larsen et al., which disclosure is incorporated by reference herein.
The signals (oximetry signals) derived from a pulse oximetry sensor are inversely proportional to the net absorption by nearby tissues of the particular wavelengths of light emitted by the sensor. The net absorption at different wavelengths is dependent upon tissue site characteristics, and includes absorption by skin pigmentation, bloodless components such as bone, non-pulsating blood, especially venous, and pulsating blood, predominantly arterial. The signal corresponding to detected light of a given wavelength is thus composed of a baseline or DC component due to non-pulsating absorption, and a pulsating or AC component related primarily to absorption by arterial blood. It is important to note that the light intensity of a given wavelength reaching the detector, and the path the light takes to get there, are determined not only by absorption properties but also by the scattering of light in tissue.
Pulse oximetry sensors typically operate in transmissive mode, wherein light from the emitters passes through one side of a vascularized tissue to reach a detector(s) on the other side of the tissue. This mode is commonly used in neonatal and adult monitoring on fingertips, earlobes, and so forth. Alternatively, the emitters and detector(s) may be placed near each other in a co-planar fashion on the same tissue surface, forming a single active site on the sensor. The light emitted by the sensor enters the tissue proximal to it and, by backscattering, returns in part to the same tissue surface and to the detector(s) of the sensor. Thus an oximetry sensor can operate in a backscattering mode, also known as reflectance mode. The oximetry signals acquired by backscattering are of lower intensity than those obtained in transmissive mode, making this method more susceptible to interference from various sources.
Pulsations occurring in synchrony with the heart rate are apparent in the oximetry signals. These pulsations result from the increased absorption of light occurring during passage of blood through the arterial system. Because the arterial pulsation is the result of systole in the heart, this rapid increase in absorption (decrease in detected light intensity) is referred to herein as the systolic phase of the signal. The period between systolic phases, characterized by a more gradual decrease in absorption, is herein referred to as the diastolic phase. The high pass filtered oximetry signal is commonly inverted when displayed as a photoplethysmographic waveform (i.e., rising with increasing absorption), emphasizing the similarity to an arterial pressure waveform.
The bulk of oxygen transport in the blood takes place bound to the hemoglobin molecule. The oxygenated (HbO2) and reduced (Hb) forms of hemoglobin have different optical extinction (absorption) curves, but blood""s scattering of light is relatively insensitive to oxygen saturation. By choosing appropriate wavelengths of light, the plurality of oximetry signals can be interpreted to yield the percentage of saturation of the hemoglobin molecules with oxygen (SpO2). In the prior art, red and infrared pulsatile amplitudes, scaled by their respective baseline or DC light intensities, are combined in a ratiometric equation based upon the Beer-Lambert model of light absorption by media to yield a ratio R related to SpO2. Most commonly, the red wavelength is nominally around 660 nm, and the infrared wavelength is in the range of 880-940 nm.
The relationship of the ratio R to SpO2 predicted by the Beer-Lambert model is actually a poor fit to empirical data. Therefore, the relationship SpO2=ƒ (R) is typically established by calibration of the oximetry system against a standard measurement of arterial oxygen saturation (SaO2). The subjects used to perform such calibration must be hypoxic to some degree, either due to a clinical condition or a laboratory procedure, in order to establish SpO2 accuracy below the typical range of a subject""s oxygen saturation. Calibration accommodates the tissue and sensor characteristics, by and large correcting the simplifications resulting from the underlying assumption of a Beer-Lambert model, which disregards the scattering of light by blood and tissue.
One application of this invention is in utero fetal pulse oximetry. The fetal oxygen sensor is inserted in or near the uterus of a mother to noninvasively monitor the condition of a fetus. One particular example of a sensor designed for fetal pulse oximetry is described in U.S. Pat. No. 5,425,362, to Siker et al. incorporated by reference herein. The sensor placement is made through the birth canal to reach a monitoring position on the fetus. This process and its outcome are difficult to satisfactorily visualize, even utilizing intrauterine imaging technologies, such as ultrasound. The fetal oxygen sensor operates in reflectance mode, a method that typically results in lower signal amplitudes, and may be subject to xe2x80x9clight shuntingxe2x80x9d, in which emitted light returns to the detector without traversing the vascularized tissue bed. Thus, fetal pulse oximetry represents a challenging scenario for signal acquisition in medical monitoring.
Even with empirical calibration, oximeter performance differs from the Beer-Lambert prediction when the characteristics of the tissue at the monitoring site vary from the characteristics at the time of calibration. Edema, or the presence of significant extracellular fluid, can result in lowered oxygen saturation readings in neonates, as described by Johnson et al., xe2x80x9cThe effect of caput seccedaneum on oxygen saturation measurementsxe2x80x9d. Br. J. Obs. and Gyn. 1990; 97: 493-498. Inaccuracies are particularly evident in conventional pulse oximeters required to operate in low oxygen saturation ranges, e.g., below 75%. In explaining the effects of edema in clinical monitoring of neonates, Johnson et al. (1990) supra also cited changes in photon path length due to increased red absorption as the explanation for lowered saturations. Severinghaus et al., xe2x80x9cEffect of anemia on pulse oximeter accuracy at low saturationsxe2x80x9d. J. Clin Mon. (1990); 6: 85-88, reported that pulse oximetry underestimated the oxygen saturation of anemic patients.
The purpose of fetal pulse oximetry is to reduce the likelihood of fetal morbidity or mortality related to hypoxia, apparent as acidosis at birth. The typical oxygen saturation level to be monitored in the fetus is below 70%, and thus calibration deviation is a concern. The term xe2x80x9ccalibration deviationxe2x80x9d refers to inaccuracies in oxygen saturation determination by a pulse oximeter specifically due to a change in the calibration, or relationship between a ratio of normalized pulse amplitudes, R, and the SpO2. Furthermore, Seelbach-Gobel et al., xe2x80x9cThe prediction of fetal acidosis by means of intrapartum fetal pulse oximetryxe2x80x9d. Am. J. Obs. and Gyn. 1999, 180(1): 73-81, present evidence that the level of fetal oxygen saturation correlating to an acidotic arterial pH level of clinical interest is below 40%. Anemia, exsanguination of blood due to pressure on the sensor, and local perfusion changes can all potentially cause a change in the blood fraction of the tissue being illuminated by the fetal oxygen sensor. Interfering factors such as caput, meconium, and extravasated blood can also cause problems.
More sophisticated models of the behavior of light in living tissue have been formulated by Takatani et al., xe2x80x9cTheoretical Analysis of Diffuse Reflectance from a Two-Layer Tissue Modelxe2x80x9d, IEEE Trans. Biom. Eng. 1979, 26: 656-664; Steinke et al., xe2x80x9cRole of Light Scattering in Whole Blood Oximertyxe2x80x9d, IEEE Trans. Biom. Eng. 1986; 33(3): 294-301; Schmitt, xe2x80x9cSimple photon diffusion analysis of the effects of multiple scattering on pulse oximetryxe2x80x9d, IEEE Trans. Biom. Eng. 1991, 38: 1194-1203; and others. Many of these models are based upon photon diffusion theory, drawing upon the earlier work of Longini et al., xe2x80x9cA note on the theory of backscattering of light by living tissuexe2x80x9d, IEEE Trans. Biom. Eng. 1968, 15: 4-10. The effects of light scattering that are not taken into account by the simpler Beer-Lambert model are incorporated to more accurately model the behavior of light in tissue. Photon diffusion models illustrate the dependence of the pulse oximeter""s calibration upon sensor design choices such as wavelength and emitter-detector spacing, as well as tissue site characteristics, particularly blood fraction or hemoglobin concentration in tissue.
Empirical calibration is still of value in a pulse oximeter, despite the improved understanding obtained by utilizing a better model. Although an equation relating SpO2 to the ratio R can be obtained, it is dependent on the other tissue site characteristics such as blood fraction, arterial versus venous blood proportion, and hematocrit. It is easier to establish the relationship by empirically measuring oxygen saturation and relating it to the ratio R.
The sensitivity of oximetry to hematocrit was noted by Schmitt, xe2x80x9cNew methods for whole blood oximetryxe2x80x9d, Ann. Biomed. Eng. 1986, 14(1): 35-52; and Steinke et al., xe2x80x9cReflectance measurements of hematocrit and oxyhomoglobinxe2x80x9d, Am. J. Physiol. 1987, 253: (Heart Circ. Physiol. 22); H147-H153. The former worked on in vivo oximetry within an artery, the latter on an in vitro instrument for analysis of blood samples. Both proposed the use of multiple light emitting devices at one isobestic wavelength (approximately 810 nm) with different spacing from the detector to measure the hematocrit. This blood fraction information could then be applied to correct the SaO2 measurement made by the same sensor.
U.S. Pat. No. 6,064,474, to Lee et al. reveals an optical method for obtaining the hematocrit value of an in vitro whole blood sample, utilizing multiple isobestic wavelengths (506 nm and 805 nm) to eliminate the sensitivity of the reading to oxygen saturation level as well as plasma scattering. The use of only isobestic wavelengths substantially simplifies the mathematical treatment, eliminating the need for empirical calibration.
U.S. Pat. No. 6,181,958 B1 to Steuer et al., reveals a system for in vivo determination of hematocrit, but implies an application in pulse oximetry as well. Photon diffusion analysis is performed on a model consisting of whole blood, water, and other tissue, with results substantially similar to Schmitt""s (1986). An expression is derived for hematocrit assuming use of an isobestic wavelength (805 nm), and it is suggested that the combination of 805 and 660 is helpful for removing pulse oximetry""s dependence on hematocrit, blood volume, and emitter-detector spacing. However, the actual solution requires precise measurement of very small pressure variations in the tissue site, utilizing an integral strain gauge, piezoelectric transducer, or other means.
U.S. Pat. No. 5,421,329 (""329) to Casciani et al. and U.S. Pat. No. 5,782,237, (""237) to Casciani et al. teach using an optimizing technique for selecting wavelength pairs having xe2x80x9cgood balancexe2x80x9d or xe2x80x9ccorrelationxe2x80x9d between the product of the absorption and scattering coefficients of each of the wavelength pairs and an optimization for spacing between the emitter and the detector to minimize sensitivity to perturbation induced artifact. Using the photon diffusion model, equations, and coefficients of Schmitt (1991) supra, describing tissue site characteristics, the ""237 and ""329 patents duplicate Schmitt""s method of perturbing the coefficients of one or more tissue site characteristics to predict the resulting calibration deviations, teaching that selecting a longer red wavelength (in the range of 700-790 nm) can reduce the calibration deviation resulting from blood fraction change to an error level considered acceptable by the inventors. The patents (""329 and ""237) address the question of reduced blood fraction, and perturbation induced artifact, such as variations in tissue composition, variations in hemoglobin concentration and variations in force applied between the tissue and the sensor. The ""237 patent cites Bonner et al., xe2x80x9cModel for photon migration in turbid biological mediaxe2x80x9d, J. Opt. Soc. Am. A 1987, 4: 423-432; and Weiss et al., xe2x80x9cStatistics of penetration depth of photons re-emitted from irradiated tissuexe2x80x9d, J. Mod. Opt. 1989, 36: 349-359, in xe2x80x9cother publicationsxe2x80x9d on the patent. The claims made in the ""329 and ""237 patents are further described in Mannheimer et al, xe2x80x9cWavelength selection for low saturation pulse oximetryxe2x80x9d, IEEE Trans. Biom. Eng. 1997, 44: 148-158; and Mannheimer et al, xe2x80x9cPhysio-Optical considerations in the design of fetal pulse oximetry sensorsxe2x80x9d, Euro. J. Obs. and Gyn. Repr. Bio. 1997, 72 (1): S9-S19.
However, choosing a wavelength which reduces calibration deviation in the available range of about 600-1000 nm invariably results in a reduction of the device""s sensitivity to oxygen saturation change. The sensitivity, equivalent to how much the ratio R varies over the full oxygen saturation range of 0-100%, is close to optimal around 660 nm. Also, for oxygen saturations above 80%, the potential error due to calibration deviation may increase for red wavelengths longer than 660 nm (contrast ""237 FIG. 8B and FIG. 12B). Apparently, for these reasons, the inventors of ""329 and ""237 recommended choosing 735 nm only for monitoring lower oxygen saturations, below 70%.
Foreign Patent Document WO 00/02483, to Tobler et al. discloses a means of detecting a change in blood fraction in a fetal pulse oximeter. The patent teaches that the oxygen saturation values may be calculated independently in an oximeter incorporating and calibrated to use several different red wavelengths. If the oxygen saturation values disagree, then the blood fraction has varied from the conditions found during calibration. However, no means is noted for correcting the deviation.
U.S. Pat. No. 5,413,100, to Barthelemy et al. discloses a method for correcting oxygen saturation measurement in the presence of carbon monoxide, teaching the use of 660, 750 and 940 nm wavelengths in a pulse oximetry sensor. The measurements from the three wavelengths are combined in a system of simultaneous linear equations to solve for the fraction of hemoglobin bound to carbon monoxide, as well as the fractions of oxygenated and reduced hemoglobin. This pulse oximeter is still susceptible to calibration deviation resulting from perturbations in tissue site characteristics.
A need therefore remains for an oximetry system in which calibration is stabilized without sacrificing sensitivity to oxygen saturation. This is especially true in the challenging domain of fetal pulse oximetry. Furthermore, for convenience it is desirable after birth to continue monitoring with the same oxygen sensor employed in utero, although the newborn""s oxygen saturation may rapidly rise above 70%. Lastly, it is preferable to further reduce the errors in oxygen saturation resulting from calibration deviation, and potentially identify and correct deviations caused by a broader range of non-ideal tissue site characteristics.
It is therefore an object of the invention to provide a pulse oximetry system with the capability of quantifying and/or eliminating the calibration deviation resulting from changes in tissue site characteristics in the pulse oximetry signals.
It is yet another object of the invention to provide a pulse oximetry system in which calibration stability can be achieved without sacrificing sensitivity to oxygen saturation changes.
It is still another object of the invention to provide a pulse oximetry system in which calibration stability and sensitivity to oxygen saturation change can both be achieved over the full oxygen saturation range for the subject.
It is yet another object of the invention to provide a pulse oximetry system with the aforementioned advantages suitable for monitoring a fetus while in utero, as well as after birth.
In one preferred embodiment of the invention, the pulse oximeter system comprises a monitor and an oxygen sensor comprising emitter(s) creating a plurality of incident light wavelengths, plus one or more light detector(s), providing signals corresponding to light returned from the tissue near the sensor after exposure to the incident light wavelengths, the system being designed to quantify and/or correct calibration deviation(s).
In another preferred embodiment of the invention, the wavelengths emitted by the oxygen sensor are chosen such that (a) at least one combination of wavelengths (a ratio wavelength set) possesses high sensitivity to blood oxygen saturation changes, and (b) at least two wavelengths (a correction wavelength set), of which at least one wavelength is not already employed in the ratio wavelength set, exhibit substantially different dependence upon a tissue site characteristic other than blood oxygen saturation.
In still another preferred embodiment of the invention, the monitor processes the signal data acquired for each wavelength in order to recalculate the signals of the ratio wavelength set, based upon both a (possibly perturbed) signal data of the ratio wavelength set, and a (possibly perturbed) signal data of the correction wavelength set.
In yet another preferred embodiment of the invention, the wavelengths of the oxygen sensor are chosen so that the calibration deviation resulting from changes in multiple tissue site characteristics can be quantified and/or corrected by employing multiple correction wavelength sets.
In another preferred embodiment of the invention, the monitor determines which correction wavelength set(s), if any, to employ in recalculating the signal data corresponding to the ratio wavelength set, based upon an estimate of the blood oxygen saturation level derived from the (possibly perturbed) signal data corresponding to the ratio wavelength set.
In still another preferred embodiment of the invention, the monitor determines which correction wavelength set(s), if any, to employ in recalculating the signal data corresponding to ratio wavelength set, based upon a pattern of a magnitude(s) and direction(s) of deviation(s) in signal data corresponding to a correction wavelength set from the expected mathematical relationships between members of each set.